In the conventional art, many semiconductor device fabrication processes utilize planar surfaces. Furthermore, as semiconductor fabrication technology progresses, increasing carrier mobility and decreasing lattice dislocation density become increasingly critical. Improving device yields by reducing dislocations provides for improved manufacturing efficiencies and cost.
In the conventional art, a silicon layer is used as the active device medium upon which semiconductor devices are fabricated. Single-crystal silicon has a specific carrier mobility value that is fundamental to the material. The mobility value is a key parameter in many active semiconductor devices. Often, it is desired to enhance or increase the device carrier mobility value to increase the switching speed and therefore the performance of the fabricated devices such as transistors. Because of the many fundamental and specific advantages in utilizing silicon as the semiconductor material, it is highly desirable to adopt methods to enhance silicon mobility instead of utilizing higher mobility materials that are harder to process such as Germanium or Gallium Arsenide.
One practical method of enhancing silicon mobility is by straining the silicon layer. By placing the active silicon under tension, significantly higher mobility resulting in higher device switching speed and drive currents can be achieved.
A method of generating such tensile strained silicon involves growing the silicon layer epitaxially above a relaxed silicon germanium film of a specific composition. This effect occurs because the silicon lattice constant, about 5.43 Angstroms, is smaller than the lattice constant of a fully relaxed silicon germanium alloy film. Such alloys can be engineered to have a lattice spacing linearly varying from 5.43 Angstroms (100% silicon) to 5.65 Angstroms (100% germanium). For the pure germanium film, the lattice spacing is about 4% larger than pure silicon. Thus for example, a Si0.75Ge0.25 alloy (25% germanium content) would have a lattice constant about 1% larger than silicon.
The strained silicon film could therefore be advantageously fabricated by epitaxially growing the device silicon film on a relaxed silicon Germanium (SiGe) alloy film of the requisite composition.
A fundamental complication of this mobility enhancement approach is the requirement of a relaxed SiGe film. If the SiGe film is grown onto a base silicon wafer, the film will first grow in a lattice-matched manner as a compressive layer. This means that the SiGe alloy will be compressed to the natural silicon lattice spacing and will be strained. Since the function of the alloy film requires a relaxation of the compressive strain, there must be a step where the SiGe alloy is relaxed to its unstrained state. Such a step necessarily introduces numerous dislocations in the SiGe layer to accommodate the lattice spacing and volume increase. The film also usually “buckles” and roughens significantly during this relaxation process.
The major parameters characterizing a practical relaxed SiGe alloy film include the amount that the film has been relaxed from its strained state (i.e. 50% relaxation would mean that the film has relaxed half of its strain), the roughness of the film, and the dislocation defect density that would be affecting the subsequent growth of the strained silicon device film.
The surface dislocation density is a critical parameter affecting the electrical properties of semiconductor materials since they are highly dependent upon crystalline defects. Dislocations can comprise insertion of an extra half-plane of atoms into a regular crystal structure, displacement of whole rows of atoms from their regular lattice position, and/or displacement of one portion of the crystal relative to another portion of the crystal. Dislocations present on the device layer can tend to short-circuit p-n junctions and also scatter electrons in a uniform n-type crystal, impeding their motion and reducing their mobility. Dislocations also cause highly localized distortion of the crystal lattice leading to the formation of “trapping” sites where the recombination of positive (holes) and negative (electrons) carriers is enhanced. This may cause, for example, the electrons from the n-p-n transistor emitter to recombine with holes in the p-type base regions before they can be collected at the n-type collector region, reducing the transistor current gain. This electron “lifetime” may be significantly reduced by recombination when as few as one out of 1011 atoms/cm3 of silicon are removed from their normal lattice sites. Although some dislocations can be removed from a semiconductor material by thermal annealing, many dislocations are permanent and thermally stable. Many of the relaxation approaches are therefore tuned to minimize the defect density of the type that can be translated to the device layer and cause device performance degradation, failure and yield losses.
In one method according to the conventional art, the SiGe alloy is grown with a slowly varying grade from 0% germanium to the required alloy composition at a sufficiently low temperature to grow a dislocation free initial film and through subsequent annealing, the slow gradient helps to accommodate film relaxation through the generation of dislocations that are buried within the SiGe layer. This technology is explained in Legoues & al. (U.S. Pat. No. 5,659,187 “Low defect density/arbitrary lattice constant heteroepitaxial layers”). To limit the production of dislocations threading to the surface, the SiGe grade is usually less than 2% composition increase per 1000 Angstroms of SiGe film growth. This shallow gradient approach is lower in productivity due to its relatively thick SiGe layer composition and may require numerous growth/anneal cycles to achieve roughness and dislocation goals.
In yet another method according to the conventional art, the surface roughness or the SiGe alloy layer can be reduced using a chemical mechanical polishing (CMP) process such as taught by Fitzgerald (U.S. Pat. No. 6,291,321 “Controlling threading dislocation densities in Ge on Si using graded GeSi layers and planarization” and U.S. Pat. No. 6,107,653, “Controlling threading dislocation densities in Ge on Si using graded GeSi layers and planarization”). CMP utilizes a combination of vertical force between a wafer and an abrasive pad as well as a chemical action of a slurry, to polish the surface of the wafer to a highly planar state. The roughness of the resulting semiconductor surface can typically be reduced to approximately 1 Angstrom RMS when measured by an Atomic Force Microscope (AFM). However, CMP is relatively costly as a result of the slurry and the amount of time it takes to perform the process. Furthermore, the CMP process does not generally reduce the dislocation density in the wafer. Finally, this linear growth/anneal/CMP sequence is costly as it requires numerous sequential process and wafer handling steps.
Another method uses miscut wafers to help the grown film to relax as much as possible and accommodate the lattice mismatches. See for example Fitzgerald & al. (U.S. Pat. No. 6,039,803, “Utilization of miscut substrates to improve relaxed graded silicon-germanium and germanium layers on silicon”) that teaches the improvement of using base wafers having 1 to about 8 degrees of miscut from a true [100] orientation to help grow a less defective, relaxed layer of a second semiconductor material. Although the base substrate miscut can improve the relaxed defect density to some extent, the improvements are generally considered insufficient for leading edge applications.
Referring to FIG. 1, a flow diagram of a process according to the conventional art is shown. This process produces a relaxed film of SiGe alloy material by first growing a strained film on a base wafer 110, subjecting the strained film to an anneal step to relax the film and concurrent generation of surface roughening (buckling) and dislocations 120, followed by a planarization smoothing step such as CMP 130. The use of an epitaxial step such as CVD (Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy) followed by a planarization step such as CMP significantly complicates the film relaxation preparation process since multiple equipment, cleans, and wafer handling are required. This in turn would increase the manufacturing cost of the relaxed film fabrication process.
Referring now to FIGS. 2A–2C, various sectional views of a semiconductor layer are shown to illustrate the anneal/CMP conventional art such as disclosed by Fitzgerald in more detail. As depicted in FIG. 2A, a single crystalline semiconductor surface formed by an epitaxial process wherein a strained SiGe film 210 is grown onto a base silicon wafer 220. The semiconductor layer is comprised of single crystalline silicon-germanium having a surface roughness 230 of approximately 1–2 Angstrom RMS. The silicon-germanium layer typically was grown at a sufficiently low temperature where the film is supercritically stressed but no relaxation has taken place. The dislocation defect density 240 is therefore very low, on the order of 1 dislocations/cm2 or less.
As depicted in FIG. 2B, an anneal is performed on the substrate to relax the SiGe alloy film which generates substantial surface roughening 250 and dislocation defects 260. The resulting surface may have a buckled roughening 250 exceeding 200–300 Angstroms RMS and a dislocation defect density 260 exceeding approximately 107 dislocations/cm2.
As depicted in FIG. 2C, a separate CMP process generally reduces surface roughness 270 to approximately 1–5 Angstroms RMS. However, the CMP process generally does not decrease dislocations 260 in the silicon-germanium layer 310 and must be accompanied by comprehensive clean processes.
Thus, the conventional art is disadvantageous in that planarizing processes are relatively costly and time-consuming processes. The conventional art also suffers from relatively high levels of dislocations. A better and less costly approach that can fully relax strained SiGe alloy films while controlling surface roughness and dislocation defect levels is highly desirable.